1. Field of the Invention
The present invention relates to a handover apparatus and method in a wireless communication system. More particularly, the present invention relates to an apparatus and method for hard handover in a wireless communication system.
2. Description of the Related Art
Wireless communication systems have been developed to perform communications regardless of the location of the user. A mobile communication system is an example of a wireless communication system. Early mobile communication systems included a system that distinguishes users based on a Code Division Multiple Access (CDMA) scheme and supports voice communications.
There is now an increased interest in providing users data services. Accordingly, mobile communication systems, as they have evolved, can now provide data services. As the interest in the data services has increased, there has been an increase in user demand for mobile communication systems that can support higher data rates. Therefore, a 3rd generation (3G) mobile communication system has been developed in order to provide higher-speed data services in the CDMA-based wireless communication system. 3G mobile communication systems are now partially in service.
However, the CDMA mobile communication system has almost reached its limitation in providing higher-speed data services due to its limited resources. Therefore, attempts are being made to provide mobile communication services with non-CDMA schemes. One of these attempts is to provide the communication services using an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
The typical OFDM-based technologies include IEEE 802.16e, Wireless Broadband (WiBro), and 3G Long term evolution (LTE) technologies. These communication systems, when compared with the CDMA communication system, can transmit more data at higher speed with the use of the OFDM scheme.
In order to allocate OFDM resources to a plurality of users, an Orthogonal Frequency Division Multiple Access (OFDMA) scheme should be used. However, the OFDMA-based wireless communication system may suffer from interference between the signals transmitted from neighboring base stations (BSs). That is, when the signals transmitted from the neighboring BSs are received at the same orthogonal frequency overlap, a terminal may not receive the signals or may experience a decrease in reception performance. Therefore, the existing OFDMA systems generally use a frequency reuse factor (FRF) of 3. In this case, each BS uses only ⅓ of its total available orthogonal frequency resources. That is, as BSs use a 3-sector structure, only ⅓ of the available resources are allocated to one sector, thereby preventing collision with the other sectors over the orthogonal frequencies.
However, some currently proposed OFDM based technologies, such as IEEE 802.16e, WiBro, and 3G LTE technologies, adopt a system that uses FRF=1. In this case, as described above, the receiver may be unable to communicate or my suffer a decrease in reception performance at or near the cell boundary. In order to solve these problems, various solutions are proposed.
A wireless communication system should not restrict the mobility of the user. In order to overcome the mobility restriction of the user, the wireless communication system uses handover technology. The term “handover” refers to a method for allowing a terminal to maintain its call while it moves from its BS to another BS.
With reference to FIG. 1, a description will now be made of a possible handover scenario. FIG. 1 is a conceptual diagram used for a description of a conventional handover scenario in a cellular wireless communication system.
Referring to FIG. 1, BSs 110, 120 and 130 have independent cells 111, 121 and 131 according to delivery distances of their signals, and overlapping areas that occur between coverages of the BSs 110, 120 and 130. The overlapping areas are divided into the areas where signals transmitted from 2 BSs overlap with each other, and the area where signals transmitted from 3 BSs overlap each other. Specifically, an overlapping area 101 exists between the cell 111 of the first BS 110 and the cell 121 of the second BS 120, an overlapping area 103 exists between the cell 111 of the first BS 110 and the cell 131 of the third BS 130, and an overlapping area 105 exists between the cell 121 of the second BS 120 and the cell 131 of the third BS 130. In addition, an overlapping area 107 exists between the cells 111, 121 and 131 of the 3 BSs 110, 120 and 130.
In an exemplary with reference to FIG. 1, a terminal 140 is located in the overlapping area 107 where the cells 111, 121 and 131 of the 3 BSs 110, 120 and 130 overlap each other. If the terminal 140 moves from the cell 111 of the first BS 110 to the overlapping area 107 where the cells of the 3 BSs overlap each other, the terminal 140 is in a handover situation. The handover changes according to the direction in which the terminal 140 moves.
Handover is classified into soft handover and hard handover, and a description thereof will be made below.
Soft handover occurs when a terminal in communication with a particular BS moves to a cell of another BS through a boundary of the BS. Here, the current (or old) BS with which the terminal is now communicating is called a source BS and another (or new) BS to which the terminal is moving is called a target BS. A description will now be made of a process in which the soft handover is performed. If the terminal in communication with the source BS moves to one of the areas 101, 103, 105 and 107 where a plurality of BSs commonly transmit data, two or more BSs transmit the same data in the corresponding area. If the terminal enters the coverage area of a target BS while receiving the same data from two or more BSs, the other BSs transmitting data to the terminal stop the data transmission. In this manner, the terminal receives the data only from the BS to which it belongs.
Hard handover also occurs when a terminal in communication with a source BS moves to a cell of a target BS through a boundary of the source BS. In the hard handover method, the terminal receives data only from one BS. That is, if the terminal in communication with the source BS satisfies a select condition, the source BS transmitting data to the terminal stops the data transmission to the terminal. Thereafter, the target BS transmits data to the terminal.
With reference to FIG. 2, a description will now be made of the typical soft handover method currently used in the CDMA scheme.
FIG. 2 is a graph illustrating a relationship between the received signal and time and is used for a description of a scenario where conventional soft handover is used in the CDMA scheme.
Shown in the graph of FIG. 2 are received strengths for the signals that a terminal receives from each of BSs while on the move. In FIG. 2, a BS_A is a source BS to which the terminal belongs, and a BS_B is a target BS to which the terminal will perform handover. The signal strength measured by the terminal, for the signal transmitted by each BS, is referred to as “Ec/Ior of a pilot signal.” As shown in FIG. 2, a curve 210 for the strength of a pilot signal received from the BS_A decreases with the passage of time, while a curve 220 for the strength of a pilot signal received from the BS_B increases with the passage of time. Referring to FIG. 1, this scenario occurs when the terminal 140 moves from the cell 111 of the first BS 110 to the cell 121 of the second BS 120.
As illustrated in FIG. 2, the terminal measures strengths of the pilot signals received from the BSs, and performs soft handover depending on the measurement result. A brief description of the soft handover operation will now be made.
Reference Points 1, 2, 3, 4, 5 and 6 shown on the time axis are time points given for a description of a soft handover scenario.
At Reference Point 1, the terminal is receiving service from the BS_A, and an active group of the terminal includes only BS_A. Even when the terminal is located in the cell of the BS_A, a pilot signal from the other BS may arrive at the terminal. Therefore, the terminal measures the strength of a pilot signal transmitted from the BS_B and compares the measured strength with a threshold T_ADD. If the measured strength of the pilot signal from the BS_B is greater than the threshold T_ADD, the terminal registers the BS_B in its candidate group and starts management thereof. That is, Reference Point 1 indicates the time point at which the BS_B is registered in the candidate group.
Thereafter, if the terminal continues to move to the cell of the BS_B, the strength of the pilot signal received from the BS_B becomes higher than the strength of the pilot signal received from the BS_A. After this situation, if the strength of the pilot signal from the BS_B is higher than the strength of the pilot signal from the BS_A by a given margin σ at Reference Point 2, the terminal registers the BS_B in its active group and starts management thereof. At this point, the terminal receives a handover message from the BS_A. The handover message includes BS_B information for handover, such as a Pseudo-random Noise (PN) offset and a traffic Walsh code number of the BS_B. From this time on, the terminal receives traffic signals from both the BS_A and the BS_B and soft-combines the received traffic signals. The terminal continues to manage the handover while monitoring the pilot signals from the two BSs. That is, two pilots are managed in the active group.
In the meantime, if the strength of the pilot signal from the BS_A is lower than T_DROP at Reference Point 3, the terminal starts a drop timer.
Thereafter, the strength of the pilot signal from the BS_A may become higher than T_DROP at Reference Point 4 according to a moving path of the terminal or a reception path of a signal. In this case, the terminal resets the drop timer.
Thereafter, if the terminal continues to progress to the cell of the BS_B and detects that the strength of the pilot signal from the BS_A becomes lower than T_DROP at Reference Point 5, the terminal starts the drop timer.
After the start of the drop timer, if the strength of the pilot signal from the BS_A continues to decrease and the measured time of the drop timer of the terminal arrives at a threshold T_TDROP at Reference Point 6, the terminal sends a pilot measurement result message to the BS_A, moves the pilot signal from the BS_A from the active group to a neighbor group, and then sends a handover complete message to the BS_A, completing the handover.
As described in FIG. 2, the soft handover is called “make-before-break-switching,” because switching of BSs providing a service is achieved at the boundary of two BSs without a drop of the traffic. Therefore, for the soft handover, the terminal should always detect pilot signals from neighbor BSs, measure strengths of the detected pilot signals, and manage the BSs. In addition, for serving one terminal, two BSs should be activated. In other words, the same voice data or packet data should be allocated to two BSs for a soft handover interval.
Next, a description will be made of a hard handover scenario.
FIG. 3 is a graph illustrating a relationship between the received signal and time, and is used for a description of a scenario where conventional hard handover is performed.
Shown in the graph of FIG. 3 are received strengths for the signals that a terminal receives from each of at least two BSs while on the move. BS_A is a source BS to which the terminal belongs, and a BS_B is a target BS to which the terminal will perform handover. Herein, there is no specific restriction in the signal measured by the terminal. Generally, the signal measured by the terminal can be a pilot signal, but it can also be a traffic signal according to system. Therefore, the strength of the received signal can be SNR, CINR or CIR, all of which indicates a signal-to-noise ratio. Similarly, in FIG. 3, a curve 310 for the strength of a signal received from the BS_A decreases with the passage of time, while a curve 320 for the strength of a signal received from the BS_B increases with the passage of time. Referring to FIG. 1, this hard handover scenario occurs when the terminal 140 moves from the cell 111 of the first BS 110 to the cell 121 of the second BS 120.
Reference points are provided in FIG. 3 for the convenience of description. A description will now be made of an operation performed between a terminal and BSs with reference to the reference points.
At Reference Point 1, the terminal has been receiving a service from the BS_A, and an active group of the terminal includes only the BS_A. The terminal measures the strength of a signal transmitted from the BS_A, and compares the measured strength with a threshold H/O_Threshold. At this time, the terminal may measure the strength of a signal from a neighboring BS, BS_B for example, register BS_B in a candidate group, and start management thereof. Generally, however, in the system supporting hard handover, the frequency reuse factor (FRF) or frequency reuse pattern (FRP) is set to 3, 5 or 7, and thus BSs use different frequencies. Therefore, in order to measure the strength of the signal from the BS_B, the terminal should shift a reception frequency for a predetermined time before the measurement, and then return to the frequency of the current BS_A. In some cases, the terminal does not manage the candidate group because of the load. It will be assumed herein that the terminal does not manage the candidate group. As illustrated in FIG. 3, if the strength of the signal received from the BS_A is lower than a hard handover threshold at Reference Point 1, the terminal releases the channel connected to the BS_A. That is, if the terminal moves further to the cell of the BS_B, the terminal measures the strength of the signal transmitted from the BS_A, and if the measured strength is lower than or equal to a threshold H/O_Threshold, the terminal performs handover because it can no longer receive traffic signals from the BS_A. For handover, the terminal attempts to access a BS having the highest signal strength among neighboring BSs.
However, as shown in FIG. 3, in some cases, the strength of a signal from BS_B is also not high enough to receive traffic from Reference Point 1 to Reference Point 2. In other cases, even though the signal strength is high enough, the BS_B cannot respond to a service request from the terminal because resource management of the BS_B is impossible. In this case, a no-service duration or call drop can happen, as shown in FIG. 3.
As described above, the hard handover performs BS switching in a very simple manner, but is much inferior to the soft handover in terms of handover success rate and reception stability. That is, in the hard handover, also known as “Break-before-make-switching,” if the strength of a signal from a serving BS decreases to a particular threshold, the terminal releases (or disconnects) the serving BS, and then searches for another BS. Therefore, the hard handover is generally used for FRP>1 due to the interference between neighbor cells. The CDMA soft handover is generally used for FRP=1.
As described above, the wireless communication system is being developed to transmit a larger amount of data at higher speed. The OFDMA systems are developed to meet the expectation.
A description will now be made of a WiBro or MobileWiMAX system, which is the current IEEE 802.16-based OFDMA system.
As described above, the WiBro or MobileWiMAX system, which is the IEEE 802.16-based OFDMA system, uses FRF=1. The use of FRF=1 is advantageous in that the frequency efficiency is high, but disadvantageous in that all sub-carriers in use overlap with sub-carriers of neighboring BSs, causing mutual interference. Due to the interfering signals from the neighboring BSs, the terminal located in the cell boundary may suffer a decrease in reception performance, and experience call drop during handover.
With reference to FIG. 4, a description will now be made of structures of a downlink and an uplink used in the OFDMA system.
FIG. 4 is a diagram illustrating conventional structures of a downlink and an uplink used in an OFDMA system.
Referring to FIG. 4, reference numeral 410 indicates a structure of a downlink, and reference numeral 430 indicates a structure of an uplink. The vertical direction 401 represents a plurality of orthogonal frequency resources, i.e. sub-carriers. The structure of the downlink will be described below.
A preamble 411 is located at the head of the downlink 410, and then followed by a frame control channel (FCH) 413 containing frame configuration information and synchronization information, and a Downlink MAP (DL-MAP) 415. Thereafter, an Uplink MAP (UL-MAP) 417 containing position information of the bursts to be transmitted over the uplink is transmitted. After the UL-MAP 417, DL bursts 419, 421, 423, 425 and 427 to be provided to users together with the UL-MAP 417 are transmitted.
A control channel 431 is first transmitted over the uplink 430, and then UL bursts 433, 435, 437 and 439, which are data transmitted by users to a BS, are transmitted.
In the above data, the elements affecting a data rate of the entire system include such control information as the FCH 413, the DL-MAP 415 and the UL-MAP 417. The control information needs to be correctly received, in order for the terminal to receive the transmitted frame data without error.
The frame illustrated in FIG. 4 is a frame structure for the OFDMA system using a Time Division Duplexing (TDD) scheme. That is, it is shown that the frame is divided into a downlink (DL) interval and an uplink (UL) interval in the time axis 402. As described above, a first symbol of the downlink frame is the preamble 411. The terminal uses the preamble signal for synchronization acquisition, BS ID acquisition, and channel estimation. Because a BS ID of the BS is used as a seed value for scrambling and sub-carrier permutation, the BS ID acquisition is necessary for decoding downlink data bursts. The preamble 411 is followed by the FCH 413, and the FCH 413 contains information necessary for DL-MAP decoding. That is, the FCH 413 includes such information as a length and a decoding scheme of the DL-MAP 415. The DL-MAP 415 includes information necessary for decoding DL data bursts of this frame. That is, the DL-MAP 415 includes position and size information of each burst, and Modulation and Coding Scheme (MCS) information.
A description will now be made of a general structure of a transmitter for transmitting data.
FIG. 5 is a block diagram illustrating an internal structure of a conventional data transmitter used in an OFDMA system.
Referring to FIG. 5, transmission data is input to an encoder 501. An encoder capable of performing forward error correction (FEC) is used for the encoder 501. Because such an encoder is well known in the art, a description thereof will not be provided herein. The encoder 501 encodes the input data, and outputs the coded symbol to a symbol mapper 503. The symbol mapper 503 modulates the input symbol into a QPSK/16QAM/64QAM symbol. A repeater 505 repeats the modulated symbol according to a repetition number (the number of repetitions) set by a BS. The repeated symbols are input to a sub-carrier permutator 507, and the sub-carrier permutator 507 permutes the repeated symbols into corresponding sub-carriers. In the sub-carrier permutator 507, the repeated symbols are regularly permuted according to a sub-carrier permutation rule unique to each BS, and then allocated to sub-carriers. The sub-carriers are input to a scrambler 509, and the scrambler 509 multiplies the sub-carriers by a scrambling sequence unique to each BS. An Inverse Fast Fourier Transformer (IFFT) 511 converts the sub-carriers multiplied by the scrambling sequence into a transmission signal.
The transmission signal is converted into a radio signal and then transmitted to a receiver. A structure and operation of the receiver will now be described with reference to FIG. 6.
FIG. 6 is a block diagram illustrating an internal structure of a conventional data receiver used in an OFDMA system.
Referring to FIG. 6, a received radio signal is converted into a baseband signal, and then input to a Fast Fourier Transformer (FFT) 601. The FFT 601 converts the time-domain input signal into a frequency-domain signal. A descrambler 603 descrambles the signal output from the FFT 601 through an inverse process of the scrambling process performed in the transmitter. The descrambled signal is input to a channel estimator 605 and a channel compensator 607. The channel estimator 605 estimates a channel between the transmitter and the receiver depending on the descrambled signal, and provides the channel estimation information to the channel compensator 607.
The channel compensator 607 compensates for channel distortion using the descrambled signal and the channel estimation information. The signal output from the channel compensator 607 is input to a sub-channel ordering unit 609, and the sub-channel ordering unit 609 orders a signal of sub-channels each composed of sub-carriers, and outputs to the resulting signal to a repetition combiner 611. The repetition combiner 611 combines the signals repeated in the transmitter, and a symbol demapper 613 demaps the combined signal output from the repetition combiner 611 using a demapping scheme corresponding to the mapping scheme used for transmission. A decoder 615 decodes the demapped symbols into the transmitted data. An FEC decoder is used for the decoder 615. Data transmission/reception is achieved through the above process.
In order to overcome the foregoing interference problem in the cell boundary, the IEEE 802.16 standard modulates a BS transmission signal with low-order modulation such as QPSK, applies a low FEC coding rate, and uses a repetition number=6. Despite such attempts, in the fading channel, outage probability increases, so that data is not received at the terminal receiver in the cell boundary, and handover performance also deteriorates. In order to overcome such problems, FRF=3 should be used. However, the use of FRF=3, compared with the use of FRF=1, decreases the frequency efficiency to ⅓ and increases cell planning complexity.
Therefore, various other methods can be considered in order to increase reception performance of the receiver. For example, a scheme for obtaining reception diversity by applying two or more antennas to the receiver can be considered. In this scheme, reception performance increases by 3 dB or more with the use of only 2 reception antennas. In this case, however, complexity of the receiver considerably increases, and the performance degradation due to the interference signals is only negligibly improved. In the IEEE 802.16 system, the reception performance greatly depends on whether the DL-MAP is received. Because the DL-MAP is a signal that is broadcasted to all terminals associated with a BS as described in FIG. 4, DL-MAP reception performance is only negligibly improved even with the use of Smart Antenna (SA) technology, Multiple-Input-Multiple-Output (MIMO) technology, and Hybrid Automatic Repeat Request (HARQ) technology. In addition, the reduction in reception performance in the cell boundary causes degradation of handover performance.
The latest proposed OFDM/OFDMA-based mobile communication systems use FRP=1. In this case, because FRP=1, the system can also apply CDMA soft handover. However, most OFDM/OFDMA mobile communication systems consider using hard handover for the following reasons. Because the systems are basically Internet Protocol (IP)-based services rather than the voice services, a process of soft-combining IP packets in the cell boundary is a heavy load on the infrastructure system (including BS and wired IP network). That is, it is not easy to support a function that two BSs manage packets having the same IP address and soft-combine the IP packets.
For example, in order for two BSs to transmit packets having the same IP address and soft-combine the IP packets, the two BSs should simultaneously transmit the IP packets to the terminal. However, the IP packet service is provided for transmission, scheduling of which does not guarantee continuity. Therefore, each BS has a queue, which is a kind of a packet buffer, to transmit packets according to priority. A process of simultaneously transmitting the same packets to queues of two BSs for soft combining may cause overflow of the queues, and in the worst case, bring the system down. Therefore, current systems consider using the hard handover even though there is interference between neighboring cells.
Referring back to FIG. 3, if the terminal performs handover in the course of receiving IP packets from the BS_A, it should disconnect the BS_A and then access the BS_B. At this point, the terminal re-establishes an IP network with the BS_B and re-accesses the BS_B using the same IP address. For example, the 3GPP2 cdma2000 1xEV-DV, cdma2000 1xEV-DO, and 3GPP HSDPA/HSUPA standards also consider using the hard handover, and instead, consider using Fast Cell Switching or Fast Cell Selection. In most cases, however, such systems cannot normally receive control signals for packet reception and traffic signals because the interference power in the cell boundary is very high. In particular, a loss of the control signals increases the handover failure rate and considerably decreases the system throughput. In addition, the hard handover, compared with the existing soft handover, increases handover processing time, causing a considerable reduction in quality of service (QoS) for the services having a very strict timing constraint, like the Voice over IP (VoIP) service. Accordingly, for actual system implementation, there is a strong demand for solutions to the problems in the FRP=1 system.
Accordingly, there is a need for an improved apparatus and method for handover in a wireless communication system.